1. Field of the Invention
The present invention relates to a method of forming heat exchangers using an aluminum alloy suitable for heat exchanger components, wherein the term "heat exchangers" includes condensers and evaporators for automobile cooling systems. More particularly, the present invention relates to a method of forming heat exchangers with aluminum alloys having such improved brazing characteristics as to be suited for heat exchanger components assembled by brazing with a flux of fluoride complexes.
In this Specification, the percentages are represented by weight.
2. Description of the Prior Art
The heat exchanger used as a evaporator or condenser in automobile cooling systems comprises aluminum tubular elements bent in a zigzag form, and a corrugated fins interposed between the elements. In such heat exchangers, the tubular elements are connected to coolant conductors with the use of union joints and various types of connectors.
The known heat exchangers are composed of tubular elements of pure aluminum alloy such as A1100 or A1050, and fins covered with a brazing sheet, wherein the brazing sheet is made of a core of aluminum alloy such as A3003, clad in a covering of BA4045 or BA4047 alloy. The known joint members are generally made of A7N01 alloy because of its good workability and strength (hardness). The A7N01 alloy contains 1.0 to 2.0% of Mg, 4.0 to 5.0% of Zn, and 0.20 to 0.7% of Mn, the remainders of which are aluminum and unavoidable impurities. The A7N01 alloy contains 1.2% of Al, 4.5% of Mg, and 0.15% of Mn.
Under the known methods, the tubes, conductors and fins are joined to each other by brazing. The brazing agent contains flux, such as alkali metal chlorides or alkaline earth metal chlorides. Recently, fluorine flux has been put into use because of its non-corrosive nature. For example, U.S. Pat. No. 3,951,328 discloses a flux which consists essentially of potassium fluoaluminate complexes and essentially free of unreacted potassium fluoride. Such fluorine fluxes are non-hygroscopic, chemically stable, and leave no corrosive residue after the components are brazed to each other. The use of fluorine flux eliminates the necessity of cleaning the brazed joints out of the flux residue after the brazing is finished. This is advantageous over the chlorine fluxes.
However, the problem is that though the fluorine fluxes are effective for joining the tubes to the fins, they are not suitable for joining the tubes to the joint members because of the resulting small fillets. When the fillets are small, the joint is liable to break. To overcome this difficulty, it is proposed that the tubes are first brazed to the fins with a fluorine flux in a furnace filled with a non-oxidizing gas, and then the joint members are brazed to the tubes by torch. The process requires at least two steps. This takes time, and increases the production cost.
The inventors have tried to discover why the fluorine fluxes are unsuitable for joining the tubes to the joint members, and discovered that it is due to the presence of magnesium contained therein in a relatively large quantity. The magnesium reacts with the fluorine in the fluxes, thereby negating the fluxing action of fluorine.